1. Field of Invention
This invention relates to optical amplifiers, and more particularly to multi-port optical amplifiers for optical communications systems.
2. Discussion of Related Art
Optical amplifiers are considered enabling components for bandwidth expansion in dense wavelength division multiplexed (DWDM) fiber optic communications systems. In particular, silica glass Erbium Doped Fiber Amplifiers (EDFA) exhibit many desirable attributes including high gain, low noise, negligible crosstalk and intermodulation distortion, bit-rate transparency, and polarization insensitive gain. These properties make optical fiber amplifiers superior to semiconductor devices as amplifiers in fiber optic systems. Moreover, fiber-based amplifiers do not require an optical-electrical-optical (OEO) interface, in which the optical signal is first converted into an electrical signal for amplification and that back into an optical signal, as do semiconductor devices.
In a communications system of any significant size, there is typically a distribution network that includes long communication paths and nodes where the network branches. In such a network, amplifiers are required in order to maintain the amplitude of the signal and the integrity of any data in route between a source and destination. For these amplifiers to function properly, the amplifiers must exhibit high small signal gains and/or high output saturation powers.
Application of erbium-doped optical fibers as amplifiers has received considerable attention recently because the characteristic gain bandwidth of these fibers is within the telecommunications window of 1.5 xcexcm commonly used in fiber optic communications systems. Since the announcement of a single mode Er3+ doped fiber amplifier (EDFA) in 1987 at the University of Southampton, enormous research has been performed, and more than 400 U.S. patents have been issued on fiber amplifiers.
To date, erbium fiber amplifiers use erbium doped silica fibers more than one meter long to achieve greater than 20 dB gain near the 1.54 xcexcm range. More commonly, the length of the erbium doped silica fiber is approximately 10 to 20 meters. The fiber management associated with such lengths is not practical for assembly into integrated optical components. Integration of arrays of EDFAs into low cost compact packages will be necessary for deployment into the metro, access and fiber-to-the home markets. There is a compelling need for integrated optical components that have an array of high optical gain amplifiers in compact low cost packages.
In view of the above problems, the present invention provides a compact multi-port EDFA and a method for low-cost manufacturing using fiber-drawing technologies.
This is accomplished with an array of active core elements that are coupled to an array of inputs, such as from telecom fibers or ports from a separate optical component, located at the elements"" respective input ports to receive optical signals and coupled to an array of outputs at the elements"" respective output ports to deliver amplified optical signals. The array of active core elements are embedded in an inner cladding layer, which together with a pair of outer cladding layers (air or a compatible material with a lower index) define an optical pump waveguide. Pump light is injected into the waveguide either directly into the inner cladding layer from the side through an entrance aperture or via a prism or notch, and confined within the inner cladding layer, which is substantially transparent to the pump wavelength, so that the pump light moves in a transverse direction with respect to the longitudinal orientation of the active core elements. The inner cladding layer serves both to confine the optical signal inside the active core elements and to guide the pump light. The sides of the inner cladding layer are reflective to the pump wavelength so that the pump light illuminates multiple segments of each active core element as the pump light bounces back-and-forth and moves longitudinally down the waveguide. The illumination of each active core element causes stimulated emission, hence amplifying the optical signals passing through the active core elements.
Re-directing any pump light that remains un-absorbed from the first passage through the waveguide longitudinally back up the waveguide can further enhance absorption of the pump light. This process can repeat for as long as there is available pump light. The ends of the inner cladding layer are preferably reflective at the pump wavelength but at least one end must be substantially transmissive at the signal wavelength such that there is efficient input and output coupling of the optical signals. Single pass amplification of the signal requires both input and output ports to be transmissive at the signal wavelength. Double pass amplification of the signal only requires that one port (which acts as both input and output) be transmissive at the signal wavelength, and the opposite end to be reflective for the signal wavelength.
In one particular embodiment, the inner cladding layer is formed from a phosphate glass host. The active core elements are formed by highly doping the phosphate glass host with Ytterbium and Erbium ions, which enhance the absorption of pump light and increase signal gain, respectively. Concentrations of at least 2% weight of Erbium and 10% weight of Ytterbium provide greater than 5 db gain over the C-band (1530-1565 nm) with a length of less than 10 cm.
The use of fiber drawing technology to manufacture the multi-port fiber amplifier is essential to realizing low cost devices. Kilometers of fiber can be drawn and then diced to form many amplifiers at a per unit cost that is a fraction of what could be achieved using standard waveguide fabrication technologies. Fiber drawing also supports the formation of active core elements having the high doping concentrations needed to achieve high gain in short lengths in a glass host that is transparent to the pump wavelength. Furthermore, the active core elements are easily formed with a circular cross section; hence the TE and TM polarization modes are preserved as the signal propagates through the amplifier.
A number of fiber drawing based approaches are contemplated. In a first embodiment, gain fibers including an inner cladding and active core elements are placed in a template structure, polished and sandwiched between a pair of external cladding layers to form the waveguide. Active cylindrical fibers are placed in a regular array pattern that is set by a template such as an array of v- or u-grooves made of a compatible glass. The fibers are fixed into place by small amounts of index-matched epoxy, or fused into place by heating the entire assembly until the interfaces between fiber and groove material merge together. The bonded structure is polished to a desired thickness, leaving a flat top surface. A first outer cladding structure is fixed to this polished surface. The cladding structure provides for optical confinement. The bonded structure is then polished from the other sidexe2x80x94leaving a flat bottom surface, and the inner cladding layer of desired thickness. A second outer cladding structure is fixed on this surface completing the waveguide structure. In this embodiment, the fiber""s inner cladding layer confines the optical signal inside the core element and the inner cladding layer together with the outer cladding layers guide the pump light.
In a second embodiment, each gain fiber is drawn to have a rectangular cladding around the active core element. Each gain fiber is fusion spliced to an input and/or output telecom fiber or left open-facetted for integration with other optical components. The gain fibers are then bonded together with their claddings together forming the inner cladding layer. In the bonding process, the rectangular fibers are arranged and placed on a bottom outer cladding layer. The assembly is heated, fusing together the cladding of the rectangular gain fibers into a ribbon of fibers. A top outer cladding layer is fixed on the top surface to complete the waveguide with the active cores in the guiding layer.
In a third embodiment, the inner and outer cladding structure and array of active core elements are drawn from a common preform. A preform glass structure is produced consisting of an inner cladding layer sandwiched between two outer cladding glass layers. An array of circular holes is drilled into the rectangular-shaped inner cladding layer following the practices of optical fiber pre-form fabrication. The active cylindrical cores, containing Er-doped glass, are fitted into the holes. The entire assembly is drawn, like an optical fiber. By adjusting the drawing conditions, the entire waveguide structure is produced. Alternatively, the outer cladding structure can be attached to the drawn array structure after the pulling process. In this case, the drawn piece, consisting of the inner cladding layer with active cores, is polished to a desired thickness, and the resulting guiding layer is sandwiched between two outer cladding structures to form the waveguide.
In a fourth embodiment, a single mode waveguide is formed by bonding a layer of active gain material to a layer of inner cladding material. The assembly is diced, stacked and sliced to form the inner cladding layer with the embedded array of active core elements, which is sandwiched between outer cladding layers and drawn like a fiber. The drawn assembly is coated with reflective material and diced into multiple single-mode multi-port amplifiers.
These and other features and advantages of the invention will be apparent to those skilled in the art from the following detailed description of preferred embodiments, taken together with the accompanying drawings, in which: